The present invention relates to an X-ray tube generating an X-ray by irradiating an electron beam, an anode of an X-ray target of an X-ray tube and an X-ray apparatus using the X-ray tube and, more particularly, to a medical X-ray tube and a medical X-ray apparatus which is required to be high in load resistivity and high in brightness and definition of an image.
In an X-ray generating apparatus for industrial use or medical use, an X-ray is generated by irradiating thermal electrons emitted from an cathode onto an anode target. An X-ray generating metal for the anode target (hereinafter, referred to as xe2x80x9cX-ray targetxe2x80x9d) used is tungsten (W) or a tungsten alloy which has a high X-ray generating efficiency and a high melting point.
An X-ray tube for medical use is required to produce a high definition image of a medical examination portion and to have a higher X-ray output compared to a common X-ray tube. Since most part of energy of an electron beam is converted into heat when an X-ray is generated, the X-ray target is heated to high temperature.
Further, a high power X-ray tube is so constructed that the X-ray target is rotated during electron beam irradiation in order to prevent the X-ray target from overheating. Therefore, the X-ray tube is required to have a high heat resistance and a high strength during rotation. A method for coping with this problem is disclosed, for example, in Japanese Patent Application Laid-Open No.58-59545. In the method, a tungsten or tungsten alloy layer is formed onto the surface of a molybdenum or molybdenum alloy base plate through a chemical deposition method or the like. This method has an advantage in better bonding ability between the surface of the molybdenum alloy base plate and the tungsten alloy layer and accordingly in a high thermal conductivity. A method of manufacturing an X-ray target is also disclosed in Japanese Patent Application Laid-Open No. 57-176654. In the method, a tungsten or tungsten alloy layer is successively laminated onto the surface of a molybdenum or molybdenum alloy base plate through a chemical deposition method or the like, and then the laminated X-ray target is annealed to improve the adhesive force. The X-ray tubes using such X-ray targets have a better load resistivity compared to an X-ray tube having a conventional X-ray generating metal, and can withstand a longtime and continuous use.
As the progress of an X-ray apparatus with computer processing such as a X-ray CT apparatus for medical use, an X-ray tube is required to cope with a high resolution processed image. Further, it is required that the X-ray tube can withstand a long-time and continuous use. In order to do so, it is necessary to increase input power to the X-ray tube to increase the amount of X-ray radiation. In addition to this, in order to obtain a high resolution image, it is important to converge an electron beam from a cathode small, that is, to increase the brightness by small focusing and large current density. Therefore, it is required that the X-ray target can withstand a large heat load on the electron irradiation surface. To these requirements, the method of Japanese Patent Application Laid-Open No.58-59545 has a problem in that the surface of the X-ray generating metal made of a tungsten alloy is roughed and the X-ray generating efficiency is decreased as it is used long time.
On the other hand, the method of Japanese Patent Application Laid-Open No.57-176654 has a disadvantage in that the process of manufacturing the target is complex and accordingly its manufacturing cost may be increased.
An object of the present invention is to provide an X-ray tube which is high in brightness and high in resolution, and can withstand continuous long-time use, that is, can withstand a high heat load, and to provide an X-ray apparatus such as an X-ray CT apparatus capable of obtaining a more clear image using the X-ray tube.
The object of the present invention can be attained by providing an X-ray tube generating an X-ray from a metal surface by irradiating an electron beam, wherein at least a part of an electron irradiating surface of an anode target of the X-ray tube comprises an X-ray generating metal having an average crystal grain diameter not larger than 30 xcexcm, preferably not larger than 10 xcexcm, on the surface of a base plate made of a metal. The xe2x80x9caverage crystal diameterxe2x80x9d here means a minor axis when the crystal grain is flat. The crystal grain diameter may be obtained by taking a picture of a polished surface using an optical microscope or an electron microscope, and calculating through an image processing method or measuring crystallographically using an X-ray. In these cases, although the crystal grain diameter is apt to be measured smaller in a case of using the X-ray, it is sufficient that the measured average crystal grain diameter is within the above range whichever method is chosen.
It is preferable that the X-ray generating metal having an average crystal grain diameter not larger than 30 xcexcm is composed of two or more layers. The xe2x80x9ctwo or more layersxe2x80x9d means that the composition of each layer may be different, or a boundary may be simply formed between layers. For example, in a case of forming an X-ray generating metal layer through the chemical vapor deposition method, by stopping to supply the process gas for a while during forming a layer and then starting to supply the process gas, a boundary is formed and two layers can be observed. In film forming through chemical vapor deposition, seed crystals are firstly formed on a base plate and then crystals grow based on the seed crystals to form a film. When supply of the process gas is stopped for a while, crystal growth is stopped at that time. When supply of the process gas is started again, seed crystals are newly formed. In such a way, two or more layers of metal films can be formed even if the composition of each of the layers is the same. The most convenient way to judge whether two or more layers are formed is to polish a cross section of the X-ray target and observe it by a microscope.
Further, it is preferable that, in the X-ray tube, the X-ray generating metal having an average crystal grain diameter not larger than 30 xcexcm is composed of two or more layers containing tungsten and rhenium, and tungsten concentration in the layer in contact with the metal base plate is higher than tungsten concentration in the surface layer of the electron irradiating surface. A preferable X-ray generating metal is a substance having a larger atomic number which has a higher X-ray generating efficiency, but it is required to have a higher melting point. Although tungsten is generally used as an element to satisfy these requirements, rhenium is added as an alloy element since tungsten itself is low in high temperature strength and accordingly is unsuitable for practical use.
It is also preferable that the thickness of the X-ray generating metal layer is not larger than 200 xcexcm.
It is preferable that the X-ray generating metal layer described above has a tungsten alloy layer in the side of the base plate.
Further, the present invention provides an X-ray tube in which at least a part of an electron irradiating surface of an anode target of the X-ray tube comprises two or more layers of alloy layers on the surface of a metal base plate. The definition of xe2x80x9ctwo or more layersxe2x80x9d is the same as described above.
Furthermore, the present invention provides an X-ray tube generating an X-ray from a metal surface by irradiating an electron beam in which at least a part of an electron irradiating surface of an anode target of the X-ray tube comprises an X-ray generating layer having a columnar crystal structure on the surface of a metal base plate. The xe2x80x9ccolumnar crystal structurexe2x80x9d hear means a crystal structure in which directions of crystals (directions of longitudinal axis of the crystals) are oriented in nearly the same direction and the aspect ratio of the crystal is approximately more than 5.
Further, the present invention provides an X-ray tube generating an X-ray from a metal surface by irradiating an electron beam, in which at least a part of an electron irradiating surface of an anode target of the X-ray tube comprises an X-ray generating layer made of tungsten and rhenium on the surface of a metal base plate, and concentration of elements except for the tungsten and the rhenium in the X-ray generating metal is not larger than 100 ppm. The concentration is indicated by unit of weight ratio and analyzed through a method such as chemical analysis, instrumental analysis or the like.
It is preferable that the metal layer containing tungsten and rhenium having maximum thickness of not larger than 100 xcexcm is formed at least on a part of a base plate made of a metallic sintered material having molybdenum as the main component in the side of electron irradiating surface. There is no need that the X-ray generating metal layer covers the whole surface of the electron irradiating surface of the metal base plate, but the X-ray generating metal layer may exist in, for example, a radial shape. It is preferable that a metal layer containing tungsten and rhenium having an average crystal grain diameter not smaller than 30 xcexcm is formed at least on a part of a base plate made of a metallic sintered material having molybdenum as the main component in the side of electron irradiating surface, and the metal layer having average crystal grain diameter not larger than 10 xcexcm is formed at least on a part of the metal surface having an average crystal grain diameter not smaller than 30 xcexcm in the side of electron irradiating surface. It is preferable that a clear boundary exists between the metal surface having an average crystal grain diameter not smaller than 30 xcexcm and the metal layer having average crystal grain diameter not larger than 10 xcexcm.
Further, it is preferable that the metal layer containing tungsten and rhenium is formed at least on a part of a base plate made of a metallic sintered material having molybdenum as the main component in the side of electron irradiating surface, and distribution of rhenium in the metal layer is uniform. When a cross section of an X-ray generating metal of a sintered material sintered formed by adding rhenium powder is observed by a scanning electron microscope and analyzed by an electron probe micro-analyzer, it is found that rhenium particles as it is exist in the sintered material and accordingly there is deviation in rhenium distribution. In a case of forming the metal film through a method such as chemical vapor deposition method, physical vapor deposition method, sputtering method or the like, such variation does not exist and rhenium is uniformly dispersed in the tungsten.
It is preferable that the metal layer containing tungsten and rhenium is formed at least on a part of a base plate made of a metallic sintered substance having molybdenum as the main component in the side of electron irradiating surface, and relative density to the theoretical density of the metal layer is not smaller than 98%. A value described in a chemical handbook or the like is used as the theoretical density. The density may be measured through a hydraulic replacing method (Archimedes"" method) or the like. The most convenient way to measure the density of the X-ray generating metal of metal thin film is to mechanically peel off the film from the base plate.
It is preferable that the composition ratio of rhenium to tungsten of the metal layer containing tungsten and rhenium is larger in the electron irradiated side of said layer. The efficiency of generating X-ray is larger in a metal having a larger atomic number. The atomic number of tungsten is 74 and the atomic number of rhenium is 75. Therefore, the efficiency of generating X-ray is larger in rhenium than in tungsten. On the other hand, the penetrating depth of electron into the X-ray generating metal surface is approximately 10 xcexcm, but it depends on the energy of electron. Therefore, it is preferable that the content of rhenium is made large in the zone up to the depth of 10 xcexcm from the surface and the content of tungsten is increased as the depth approaches to the metal base plate. The melting point of rhenium is lower compared to that of tungsten, and the price of rhenium is higher compared to that of tungsten. In regard to surface melt and cost, it is not preferable to make the content of rhenium excessively high.
FIG. 1 is a view showing a simulation result of temperature distribution in an X-ray target of an X-ray tube during using. Temperature at the surface of the electron irradiating surface is increased up to approximately 1500xc2x0 C., but temperature at a position beneath the surface is steeply decreased. In a case where graphite is used as the base plate and an X-ray generating metal layer is formed on the electron irradiating surface though chemical vapor deposition method, temperature at the boundary between the graphite base plate and the X-ray generating metal layer is increased above 1300xc2x0 C. since the X-ray generating metal layer is formed so as to have a thickness less than 500 xcexcm due to manufacturing cost. In such a temperature condition, the graphite reacts with the tungsten in the X-ray generating metal layer made of a tungsten-rhenium alloy to form a carbide such as tungsten carbide. When such a carbide is formed, the bonding force in the boundary is decreased, and cracks and delamination possibly occur at the junction portion during using the X-ray tube.
Since such a carbide has a small thermal conductivity, the heat generated on the electron irradiating surface is not sufficiently dispersed. That is, the temperature of the electron irradiating surface is increased and the load resistivity is decreased.
The inventors of the present invention invented the present invention by studying an X-ray target which did not decrease its load resistivity due to formation of such a carbide. That is, the inventors of the present invention found that an X-ray target having a high load resistivity could be obtained by making the base plate of the X-ray target with a metal sintered material such as molybdenum and forming an X-ray generating metal film having average grain diameter smaller than 30 xcexcm on the base plate using a thin film technology such as a chemical vapor deposition method.
There is a phenomenon that the surface shape of the X-ray generating metal is roughened when an X-ray tube is used for long time. This phenomenon is caused by sublimation or melting of the X-ray generating metal because the temperature near the electron irradiating surface increases up to approximately 2000xc2x0 C. When the surface is roughened, the X-ray generating amount is decreased because X-ray emitted from the surface of the X-ray generating surface is scattered by the rough surface. FIG. 2 is a schematic view showing this phenomenon.
The inventors found that small crystal grain diameter was effective to suppress this phenomenon. The reason is that sublimation and melting of the X-ray generating surface occur in the grain boundaries first. FIG. 3 is a schematic view showing this phenomenon.
From these facts, the inventors found that an X-ray tube had a high brightness and a small degradation in performance when it was used for a long time. The X-ray tube comprised an X-ray target of an X-ray generating metal layer having average grain diameter not larger than 30 xcexcm, preferably not larger than 10 xcexcm, formed through chemical vapor deposition method or the like.
FIG. 4 is a graph showing the relationship between crystal grain diameter and surface roughness of an X-ray generating metal layer. In order to accelerate testing time, this test was performed by irradiating YAG laser instead of electron beam to supply a high heat input and measuring worn amount of the X-ray generating metal surface. It can be understood from the result that the X-ray target having a crystal grain diameter smaller than 10 xcexcm is smaller in worn cross sectional area and smaller in surface roughness than the X-ray target having a crystal grain diameter of nearly 50 xcexcm. The reference character Z in FIG. 4 indicates the distance between the center of a laser focus lens and a sample surface. FIG. 5 is photographs showing cross-sectional features. The photograph in FIG. 5(a) shows a cross-sectional feature of the chemical vapor deposited tungsten-rhenium layer (20 go-and-return cycles), and the photograph in FIG. 5(b) shows a cross-sectional feature of the sintered tungsten-rhenium layer (20 -go-and-return cycles). The length of 1 cm in FIG. 5 corresponds to 20 xcexcm.
FIG. 6 is a graph showing dependence of crystal grain diameter in X-ray generating metal on heating temperature. It can be understood that the crystal grain diameter of an X-ray generating metal layer having initial grain diameter of nearly 1 xcexcm is grown not so large after heating at 2000xc2x0 C. for 1 hour. This means that the crystal grain diameter of the X-ray generating metal layer does not coarsen with time and accordingly there is little problem in surface roughing.
An X-ray target shown in FIG. 7 was manufactured. The X-ray target was manufactured by forming a tungsten-rhenium sintered alloy having thickness of approximately 10 xcexcm on the surface of a molybdenum sintered alloy base plate to manufacture a base X-ray target, and by further forming an X-ray generating metal layer having crystal grain diameter smaller than 10 xcexcm and thickness of 100 xcexcm on the half surface of the base X-ray target. The X-ray target was irradiated with an electron beam for a predetermined cycles while the X-ray target was being rotated, and then rotation of the target was stopped. FIG. 8 is a graph showing the measured result of amount of generated X-ray and reducing ratio of X-ray generation for the side with the X-ray generating metal layer and the side without the X-ray generating metal layer. The amount of generated X-ray is more in the side with the X-ray generating metal layer by nearly 10% than in the side without the X-ray generating metal layer. The reducing ratio of generated X-ray is less in the side with the X-ray generating metal layer by nearly 5% than in the side without the X-ray generating metal layer. FIG. 9 is photographs showing cross-sectional structures near the X-lay generating metal layers after the test. The photograph in FIG. 9(a) shows a cross-sectional feature of the chemical vapor deposited tungsten-rhenium layer, and the photograph in FIG. 9(b) shows a cross-sectional feature of the sintered tungsten-rhenium layer. The length of 1 cm in FIG. 9 corresponds to 100 xcexcm. The surface roughness is smaller in the side with the X-ray generating metal layer than in the side without the X-ray generating metal layer. Measurement by a probe type surface roughness meter showed that the average roughness (Ra) and the maximum roughness (Rmax) in the side with the X-ray generating metal layer were 5.7 xcexcm and 45 xcexcm, and on the other hand the average roughness (Ra) and the maximum roughness (Rmax) in the side without the X-ray generating metal layer were 7.5 xcexcm and 71 xcexcm. That is, the surface roughness was smaller in the side with the X-ray generating metal layer than in the side without the X-ray generating metal layer.
After studying the differences in the test results of the X-ray target with the X-ray generating metal layer and the X-ray target without the X-ray generating metal layer, the following results are obtained.
(1) When the crystal grain diameter of the electron irradiating surface is smaller than a certain value, the surface roughness is small.
(2) When there is a boundary between the surface layer and the base plate, a crack starting from a point on the surface is suppressed to progress and the crack progress distance is shortened.
(3) It is revealed from an analysis using an electron probe micro-analyzer that rhenium distribution in the X-ray generating metal layer formed on the surface is uniform compared to that in the sintered tungsten-rhenium layer.
(4) The relative density to the theoretical density is large in the surface of the X-ray generating metal layer than in the surface of the sintered tungsten-rhenium layer. That is, the sintered tungsten-rhenium layer has a lot of voids and the surface roughness is large.
Based on the above test data, the requirements for an X-ray tube having high brightness and long life-time are obtained as follows.
(1) An X-ray generating metal layer having a maximum drain diameter not larger than 30 xcexcm, preferably a maximum grain diameter not larger than 10 xcexcm, is formed on the surface of a metal base plate made of molybdenum or the like.
(2) A boundary exists between the X-ray generating metal layer and the metal base plate or inside the X-ray generating metal layer to prevent progress of a crack.
(3) Rhenium distribution in the X-ray generating metal layer is uniform.
(4) Relative density to the theoretical density in the X-ray generating metal layer is not smaller than 98%.
With the above specified construction, an X-ray tube having high brightness and long life-time can be obtained.
A method of manufacturing an X-ray generating metal layer in accordance with the present invention is characterized by that a tungsten-rhenium film of the X-ray generating metal is formed by using metal halide gases (WF6, ReF6) containing hydrogen and maintaining the base plate temperature within the range of 200 to 600xc2x0 C., preferably 400 to 500xc2x0 C., in which the film forming speed is high and a uniform fine structure can be obtained. When the base plate temperature is lower than 200xc2x0 C., the film is apt to become non-uniform. On the other hand, when the base plate temperature is higher than 600xc2x0 C., the fine structure is hardly obtained because content of rhenium becomes low. In order to make the film forming speed high, it is preferable that the chemical vapor deposition pressure is set to near atmospheric pressure. Further, it is also preferable that an amount of rhenium contained in the fine structure tungsten-rhenium alloy is in the range of 2.5 to 26 wt % in order to form the fine structure.
As for a method of manufacturing an X-ray target in accordance with the present invention, it is preferable that a fine structure tungsten-rhenium alloy as an x-ray generating metal material is coated onto a heat resistant anode base plate made of molybdenum or a molybdenum alloy, or tungsten or a tungsten alloy, or a complex base plate formed by laminating layers made of the materials, and then the coated X-ray target is performed with heat-treating at a temperature of 1000 to 2000xc2x0 C. in a vacuum environment. By the vacuum heat treatment, diffusion between the metal base plate and the X-ray generating metal coated onto the metal base plate is progressed, and at the same time gas contained in the X-ray target is completely removed. When the heating temperature is lower than 1000xc2x0 C., diffusion between the coated X-ray generating metal and the base plate made of molybdenum or the molybdenum alloy, or tungsten or the tungsten alloy, or the complex base plate formed by laminating layers made of the materials is insufficient and accordingly the coated X-ray generating metal cannot closely attached to the base plate or the complex base plate. Further, the degassing of the X-ray target is insufficient and accordingly the withstanding voltage is lowered due to gas released when the X-ray target is assembled in an X-ray tube. Therefore, an X-ray having a sufficient strength cannot be generated.